PHNOM PENH, Cambodia—Opponents of long-ruling Cambodian Prime Minister Hun Sen renewed vows to continue their challenge against results of a disputed July vote and pressure the leader to resign, after staging the country’s largest protest march in 15 years.

On Sunday, the opposition Cambodia National Rescue Party rallied roughly 100,000 supporters to parade through the nation’s capital, according to municipal officials. It was part of a rolling opposition protest over the contentious July 28 general election that was narrowly won by Mr. Hun Sen and his Cambodian People’s Party amid allegations of widespread electoral fraud.

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Opposition leaders are calling on Mr. Hun Sen, who has held power for 28 years, to resign or call for a fresh election. They say they will continue their protest for three months or until the prime minister gives in to their demands. Mr. Hun Sen rejected those calls Friday, saying he had no plans to resign or hold a new poll before the end of his five-year term.

Protesters had gathered Sunday at Freedom Park in the heart of Phnom Penh before parading through the capital’s main thoroughfares, waving opposition party colors and chanting slogans demanding political change. The boisterous and largely peaceful scenes mirrored a series of mass opposition protests in recent months.

The march was Cambodia’s largest since similar protests broke out in the wake of the 1998 elections, according to local English-language newspaper The Cambodia Daily.

“A political tsunami has started in Cambodia,” Sam Rainsy, leader of the Rescue Party, told supporters before the march, according to local media reports. “Sooner or later, the [Rescue Party’s] government will take power and deal with all your problems,” he said.

The protest entered its ninth day Monday as demonstrators continued to occupy Freedom Park, a popular rallying point for opposition groups, while Mr. Rainsy and other senior Rescue Party leaders met supporters in the southeastern province of Svay Rieng.

Phnom Penh municipal officials criticized the Rescue Party’s protest for what they called “irregularities” in areas like hygiene, noise pollution and crowd control. But they have largely allowed the demonstration to proceed with minimal security intervention.

Lawmakers-elect from Mr. Rainsy’s party have boycotted Parliament since the July vote, and criticized Mr. Hun Sen for forming a new government without opposition lawmakers. Mr. Hun Sen and ruling party officials have defended the election results, but promised to consider unspecified reforms for the election commission. They also defended their formation of a new National Assembly and government, calling on the Rescue Party to end its protests and join Parliament.

In the July 28 election, Cambodia’s fifth since the United Nations reintroduced competitive elections in 1993, Mr. Hun Sen’s party won 68 parliamentary seats to 55 for the opposition Cambodia National Rescue Party, official results show. This represented a 22-seat loss for the ruling party compared with 2008, and marked the ruling party’s worst showing since 1998.

The Rescue Party—formed last year by a merger of two opposition parties—has claimed that it won 63 seats in the 123-member National Assembly, enough for overall victory and up from 29 taken by its constituent groups five years ago. The party says it was denied victory by widespread voting fraud, and its lawmakers-elect have refused to take their parliamentary seats in protest.

The National Election Committee—which has close ties to the ruling People’s Party—has rejected calls for a United Nations-backed probe. The commission has said it found some instances of voting fraud, though not enough to sway the official result.

Meanwhile, Mr. Hun Sen has continued to govern despite the opposition’s parliamentary boycott. He has rejected claims from some legal experts who say the Constitution allows the opposition to block a new Parliament by declining to take its seats.

Mr. Hun Sen, 61, is credited with restoring stability and economic growth to his impoverished nation, but has also been accused of quieting dissent through intimidation. Political analysts say support for the prime minister has weakened amid deepening disenchantment over rural-land conflicts, high youth unemployment and widening social inequity, as well as the ruling party’s inability to connect with a growing youth demographic.

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